1. Field of the Invention
The present invention relates to a surface emission type semiconductor laser adapted to emit a laser beam in a direction perpendicular to the substrate thereof and a method of making such a semiconductor laser.
2. Description of the Related Art
A surface emission type semiconductor laser that emits a light beam in a direction perpendicular to the substrate has been proposed in place of such surface emission type semiconductor lasers as disclosed in U.S. Pat. Nos. 4,637,122; 4,856,013 and 5,084,893, these being adapted to emit a light beam in a direction parallel to the substrate.
The surface emission type laser is disclosed in Lectures of the 50-th Meeting of Applied Physics in Japan (1989), Vol. 3, pp. 909, 29a-ZG-7. In accordance with the prior art, as shown in FIG. 12, there is first provided an n-type GaAs substrate 602 on which an n-type AlGaAs/AlAs multi-layer film 603, an n-type AlGaAs cladding layer 604, a p-type GaAs active layer 605 and a p-type AlGaAs cladding layer 606 are sequentially grown and formed. The multi-layered structure is then etched while leaving a column-like part at the top thereof. The remaining column-like part is enclosed by a buried layer which is formed by sequentially growing a p-type layer 607, n-type layer 608, p-type layer 609 and p-type layer 610 all of which are of AlGaAs in liquid phase epitaxy method. Thereafter, a multi-layer dielectric film 611 is deposited on the cap layer of p-type AlGaAs 610 at the top thereof. Finally, p- and n-type ohmic electrodes 612 and 601 are formed respectively on the top and bottom end faces of the structure thus formed. In such a manner, a surface emission type semiconductor laser will be completed.
The buried layer (607-608) formed in the above manner defines a p-n junction which is used as means for preventing current from leaking to layer sections other than the active layer section.
However, by using such a p-n junction, it is difficult to provide a sufficient current restriction; and it cannot suppress any reactive current perfectly. Due to generation of heat in the component, therefore, the surface emission type semiconductor laser constructed in accordance with the prior art is impractical in that it is difficult to perform a continuous generating drive in room temperature. It is thus important to restrict the reactive current in the surface emission type semiconductor laser.
Where the buried layer is of a multi-layered structure to form a p-n junction as in the prior art, the p-n interface in the buried layer should be positioned in consideration of a position of the interface between each of the adjacent column-like grown layers. It is difficult to control the thickness of each layer in the multilayered structure. It is therefore very difficult to consistently produce surface emission type semiconductor lasers.
If a buried layer is formed around the column by the liquid phase epitaxy method as in the prior art, there is a high risk of breaking-off of the column-like part, leading to a reduced yield. The prior art was thus subject to a structural limitation in improving its characteristics.
On the other hand, U.S. Pat. No. 4,949,351 discloses another surface emission type semiconductor laser that also uses a light exit side reflection mirror as an electrode. The reflection mirror is formed by sequentially depositing Ti, Pt and Au. When these metals are used for a reflection mirror, and if the thickness of this reflection mirror becomes equal to or more than 700 .ANG., almost no transmittance can be obtained, and the reflectivity does not exceed 92%. Therefore, when the reflection mirror is used as a light exit side reflection mirror, the need for transmitting light prevents the film thickness from exceeding 500 .ANG.. In this case, the reflectively of the reflection mirror becomes about 80%. Furthermore, by annealing the good ohmic contact with the semiconductor layers, the reflectivity is further reduced. If the reflection mirror is also used as an electrode for injecting a current into a resonator, it is extremely difficult to increase the reflectivity of the mirror.
The prior art raises further problems even when it is applied to various other devices such as laser printers and the like.
For example, laser printers can have an increased freedom of design as in simplifying the optical system or in decreasing the optical path, since the source of light (semiconductor laser and so on) has a relatively large size of light spot equal to several tens .mu.m and if a light emitting element having an increased intensity of light emission is used in the laser printers.
With the surface emission type semiconductor laser constructed according to the prior art, the optical resonator is entirely buried in a material having a refractive index higher than that of the resonator. Light rays are mainly guided in the vertical direction. As a result, a spot of light emission in the basic generation mode will have a diameter equal to about 2 .mu.m even if the shape of the resonator is modified in the horizontal direction.
It has been proposed that the light spots be located close to each other up to about 2 .mu.m and that a plurality of light sources be used to increase the size of a spot. From the standpoint of reproductiveness and yield, however, it is very difficult with the prior art to bury a plurality of resonators spaced away from one another by several microns using the LPE (Liquid Phase Epitaxy) method. Even if such a burying can be successfully carried out, the spots cannot be united into a single spot since the transverse leakage of light is little.
It is also necessary that a plurality of light spots are formed into a single beam of light and that the laser beams each consisted of plural spots are in phase to increase the intensity of light emission. However, the prior art could not produce a surface emission type semiconductor laser which emits a plurality of laser beams close to one another up to a distance by which one of the laser beams are influenced by the other, in order to synchronize the laser beams in phase.